Patent Document 1 discloses, for the purpose of easily improving the fatigue strength of mechanical parts or components, a carburizing treatment process on a gear, a nitriding treatment process on the gear, a first shot peening process for shot peening the gear using shot particles with a shot particle diameter of 0.8 mm, and a second shot peening process for shot peening the gear using shot particles with a particle diameter of 0.1 mm.
Patent Document 2 discloses, for the purpose of modifying the surface of alloy steel for mechanical structure, a first shot peening process for shot peening using shot particles with a shot particle diameter of 0.6 mm, which is conducted after a vacuum carburizing treatment and a heating and rapid cooling treatment for ultrahigh-speed and short time, and a second shot peening process for shot peening using shot particles with a shot particle diameter of 0.08 mm.
In the techniques of Patent Documents 1 and 2, the residual stress in a region located at a depth of 20 μm or less from the surface is adjusted to −1400 MPa to increase tooth-root bending fatigue strength. That is, the stress (compressive stress) of −1400 MPa or less is left in the region at a depth of 20 μm or less from the surface. When repeated stress on teeth acts as a large repeated stress on the surface(s) of a tooth or teeth root(s), this repeated stress is canceled out by the residual stress. Thus, fatigue strength is increased.